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Summary BOT 2601 LEARNING UNIT 2
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- University Of South Africa (Unisa)
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BOT 2601 LEARNING UNIT 2THE ORIGIN AND STRUCTURE OF THE PRIMARY PLANT BODY
Textbook reference: chapter 2 pg. 51-116
Different types of primary tissues:
- Protoderm (gives rise to epidermis)
- Procambium (gives rise to primary vascular tissue)
- Ground meristem (gives rise to ground tissue)
STRUCTURE AND FUNCTION OF THE ANGIOSPERM VEGETATIVE SHOOT APEX:
o Meristems are regions with active cell division. The cells, tissues and organs that make up the primary plant body
are formed from the apical meristems of the root and shoot.
o During early development of plant embryos, all the cells divide actively
o With continued growth and development, cell division is limited to specific regions where the cells remain
embryonic and little differentiation occurs, and the cells retain their ability to undergo division
o All embryonic tissues in the plant body are called meristems
o The shoot apical meristem is typically a dome-shaped or flattened collection of permanently embryonic cells at
the tip of the stem. It has the capacity for unlimited growth.
o No laterally positioned structures form on the shoot apex above the youngest leaf primordia
o The term shoot apex includes the apical meristem and the subapical zone where leaf primordia are initiated
o Functions of shoot apical meristem:
- Adds new tissues to the stem
- Ensures specific tissues differentiate
- Gives rise to lateral organs
o Growth of stems may be brought about by primary or
secondary growth:
- Primary growth constitutes the addition of stem
tissues consisting of cells produced by the apical
meristem
- Secondary growth occurs when tissues are
produced by meristems other than the apical
meristem such as cells formed from the cambial
zones
SHOOT APEX ORGANISATION:
o There are currently 2 views of angiosperm shoot apex organisation
o One describes the apex as a stratified region of clonally distinct cells that is composed of one or more superficial
layers that extend across the meristem to enclose a subsurface region. The outer cell layers constitute the tunica
and undergo surface growth. The tunica is composed of cells that have a common orientation and undergo
anticlinal divisions in which the wall separating 2 daughter cells is formed at right angles to the surface. The
tunica is typically 2-3 cell layers deep. These layers are named from the outside in; L1, L2, L3. The inner layers of
the tunica give rise to the corpus and eventually differentiate into vascular tissue. The subsurface mass of cells,
called the corpus, is unlayered and undergoes growth in volume
o A second, more recent interpretation of the shoot apex recognises definable zones and is referred to as
cytohistological zonation. Zone boundaries in actively dividing meristems are distinguished by differences in cell
size and shape.
- The initial/central zone represents a group of enlarged initial cells at the summit of the stem apex. These
cells undergo frequent cell division, possess prominent nuclei and are highly vacuolated. This zone functions
as a source of all other cells of the apex and the entire primary shoot
- The flanking peripheral zone is derived from the central zone and encircles the apical region. Cells in this
zone are smaller and mitotically more active and have denser cytoplasm. They form the sites of origin of the
cortex and leaf primordia that arise in ordered ontogenetic patterns.
- The transition/rib zone is located at the base of the apical meristem. This zone arises directly from the
central zone and is an intermediate region between the initial cells and partially differentiated derivative
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